How to Query Multiple Results with Dapper ORM Efficiently

Dapper is a powerful and lightweight Object-Relational Mapping (ORM) tool for C#. It is designed to be simple and fast, allowing developers to execute SQL queries quickly and efficiently. One of the key features of Dapper is the ability to execute multiple SQL statements and retrieve multiple results. In this article, we will discuss how to execute multiple results using Dapper in C#.

Before we dive into the specifics of executing multiple results with Dapper, let’s first discuss why you might need to do this. In some cases, a single SQL query might not be enough to retrieve all the data you need.

Use Case

For example, you might need to retrieve data from multiple tables or join data from different sources. In these cases, you can use Dapper to execute multiple SQL statements and retrieve multiple results.

Getting Started

Step 1: Define the SQL Statements

The first step in executing multiple results using Dapper is to define the SQL statements that you want to execute. You can define these statements as separate strings, or you can use a single string and separate the statements with semicolons.

For example, let’s say we want to execute two SQL statements:

SELECT * FROM Customers;
SELECT * FROM Orders;

Step 2: Create the Connection

Next, you need to create a connection to the database. You can do this using the SqlConnection class in the System.Data.SqlClient namespace.

using (var connection = new SqlConnection(connectionString))
{
    // ...
}

Step 3: Execute the Query

To execute the query, you can use the QueryMultiple() method of the Dapper.SqlMapper class. This method takes two arguments: the SQL statements to execute, and any parameters that you need to pass to the query.

using (var connection = new SqlConnection(connectionString))
{
    var results = connection.QueryMultiple(
        @"SELECT * FROM Customers;
          SELECT * FROM Orders;");
}

Step 4: Retrieve the Results

Once you have executed the query, you can retrieve the results using the Read() method of the returned object. This method returns a boolean value that indicates whether there are any more results to read.

using (var connection = new SqlConnection(connectionString))
{
    var results = connection.QueryMultiple(
        @"SELECT * FROM Customers;
          SELECT * FROM Orders;");

    var customers = results.Read<Customer>().ToList();
    var orders = results.Read<Order>().ToList();
}

In the example above, we are using the Read() method to retrieve two sets of results: customers and orders. We are then converting these results to lists using the ToList() method.

Async Method

To execute the query asynchronously, we can use the QueryMultipleAsync() method of the Dapper.SqlMapper class. This method takes two arguments: the SQL statements to execute, and any parameters that you need to pass to the query.

using (var connection = new SqlConnection(connectionString))
{
    await connection.OpenAsync();
    var results = await connection.QueryMultipleAsync(
        @"SELECT * FROM Customers;
          SELECT * FROM Orders;");
}

Once we have executed the query asynchronously, we can retrieve the results using the ReadAsync() method of the returned object. This method returns a boolean value that indicates whether there are any more results to read.

using (var connection = new SqlConnection(connectionString))
{
    await connection.OpenAsync();
    var results = await connection.QueryMultipleAsync(
        @"SELECT * FROM Customers;
          SELECT * FROM Orders;");
    var customers = await results.ReadAsync<Customer>().ToListAsync();
    var orders = await results.ReadAsync<Order>().ToListAsync();
}

In the example above, we are using the async version of the Read() method to retrieve two sets of results: customers and orders. We are then converting these results to lists using the ToListAsync() method.

Conclusion

Executing multiple results using Dapper in C# asynchronously is a powerful feature that allows you to retrieve complex data from your database quickly and efficiently.

By defining the SQL statements, creating the connection, executing the query asynchronously, and retrieving the results asynchronously, you can take advantage of this feature and streamline your database operations with Dapper. With its simplicity and flexibility, Dapper is an excellent choice for building high-performance database applications in C#.

More about Dapper

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Dapper Best Practices: C# Developers’ Guide to Database Management

Using Dapper with Entity Framework

ORM Wars: Dapper vs EF Core

Maximizing C# Database Performance with Dapper

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